DWP's Use of MCLs, Office of Research and Standards Drinking Water Guidelines for Drinking Water (ORSGs) and US EPA Health Advisory (HA) Levels

How MassDEP Drinking Water Program uses MCLs, ORSGs, and HA Levels

Drinking water standards and guidelines for chemical contaminants are used for purposes of protecting public health, including the most sensitive members of the population such as infants and children. The MassDEP Drinking Water Program (DWP) evaluates all drinking water sample results against federal and state maximum contaminant levels (MCL) or against guidelines created by the US EPA, or MassDEP Office of Research and Standards (ORS) when no US EPA or state MCL is available. MCLs are established by US EPA for most, but not all, of the water quality parameters that are required to be tested under the Safe Drinking Water Act (SDWA). The Commonwealth usually adopts the federal MCLs but may adopt, and in some cases has adopted, stricter standards for protecting public health.

EPA-derived MCLs may not exist for some contaminants for a variety of reasons: the chemical doesn't typically occur widely enough to be of national significance; US EPA has assigned a chemical to be of lower priority for regulation; or has determined that it doesn't pose a human health risk at concentrations typically found in drinking water. Some chemicals without MCLs, however, may pose an aesthetic problem (e.g., color, odor). In this case, a guidance value termed a "secondary" standard is provided by EPA.

When a contaminant is detected or testing is required for a contaminant that has no established MCL, guidance in the form of EPA Health Advisories (HA) or MassDEP drinking water guidelines from the Office of Research and Standards (ORSG) may be used as a basis for comparison. EPA's Health Advisories are their estimates of acceptable drinking water levels for chemical substances. They are not legally enforceable federal standards, but "serve as technical guidance to assist federal, state, and local officials" when they are managing cases of drinking water contamination in order to protect public health. In the absence of such guidance, ORS has derived ORSGs for some chemicals. A list of MCLs and ORSGs and a description of the process used to derive ORSGs can be found by following this link: 2017 Standards & Guidelines for Contaminants in Massachusetts Drinking Water .

ORSGs are established using risk assessment methods consistent with those used by EPA's Office of Water when setting guidelines for chemicals in drinking water. Exposure to concentrations of chemicals having evidence of carcinogenicity is minimized as much as feasible; thus, ORSGs for these chemicals are set at the lowest practical quantitation limit (PQL) or at an excess lifetime cancer risk (ELCR) of one in one million.

In order to protect the health of the citizens of the Commonwealth and in accordance with MGL Chapter 111, Section 160, and 310 CMR 22.00, MassDEP will enforce MCL, HA, and ORSG whenever necessary. When drinking water supply testing reveals contaminant levels in excess of an MCL, HA, or ORSG, retesting is required to validate the result. If additional testing confirms that a standard has been violated or exceeded (see Massachusetts Drinking Water Regulations for specific language for MCLs), actions are required to notify the public of the situation and to bring the water supply into compliance. These actions may include operational adjustments or treatment of the water to remove or reduce the contaminant, provision of alternate water supplies and/or remediation of the source of contamination. If an ORSG or HA is exceeded, additional steps, including a case specific drinking water health risk assessment may be initiated. Depending on the specific contaminant, its potential toxicity and/or health risk assessment results, follow-up actions to reduce exposures, may be taken. In some cases, follow-up action may occur prior to retesting to protect public health. Decisions as to whether a supply that exceeds an MCL, ORSG, or HA may remain in service depend on the type of contaminant(s) found, their levels, persistence of the compound(s), other contaminants present in the water, the duration and severity of the exceedances, whether treatment is feasible, and the availability of an alternate source of drinking water.

If you have any questions on DWP's use of Office of Research and Standards Drinking Water Guidelines and US EPA Health Advisory Levels please contact the Drinking Water Program.

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